Generally, sterilization of water is essentially performed by chlorination obtained either by an electrochlorination or by an addition of Javelle water or of liquid chlorine.
Actually, surface water such as that found in seas and lagoons contains various organic elements which can proliferate and thus cause fouling of the water circulation pipes, leading to very considerable head losses, a considerable increase in the rate of clogging of the filters, or further, a clogging of the safety filters by growth of Copepoda whose eggs pass through filtering media such as prelayers. Thus, it has been found that sterilization of water can be as important as the filtration or the anticorrosion treatment to prevent deposits and, in the case of the petroleum industry, to prevent clogging of the formation into which water is injected.
Javelle water, whose concentration is expressed in chlorometric degree, exhibits drawbacks in industry for sterilization of water because it is susceptible to aging and unstable in high concentration, loss of activity being able to reach 6 to 10 chlorometric degrees during the first 24 hours after production. Because of this, it is necessary to provide fresh Javelle water several times per week.
Liquid chlorine is still more difficult to handle because of injection systems that are relatively complex to put in place, not taking into account the safety problems which necessitate taking great precautions. Also, the shipping, handling and storing of empty bottles that have contained liquid chlorine require the same type of precautions.
Another chlorination method is the one called electrochlorination which uses electrolysis. Electrochlorination is widely used for treating sea water despite the drawbacks due to the considerable maintenance required; however, when the salinity of the water to be treated is less than that of the sea water, the production of chlorine by the electrolysis cell lowers. For example, for saline concentrations less than 4 g/l, the faradic output is low and the dimensioning of the chlorination unit becomes important for productions of chlorine necessary for the treatment.
In a closed-loop recycling system, for example in the treatment of swimming pool water, most generally chlorocyanuric compounds are used which exhibit the advantage of having a slow solubility in water and of containing more than 88% by weight of active chlorine, as is the case for trichlorocyanuric acid. However, the treatment of water in a closed-loop recycling system, for example in a swimming pool, requires only an adjustment of the amount of chlorine contained in the volume of water in the system. This is obtained by periodic addition and "all or nothing" operation of the chlorination cell. The amount of chlorine contained in the treated water can vary rather considerably but without exhibiting a drawback, the chlorine keeping a sufficiently long time and without significantly increasing the consumption of active products.
Trichlorocyanuric acid mixed with boric acid and with calcium stearate is also used for the treatment of municipal waste water, as is described in U.S. Pat. No. 4,054,518. In this patent, the compound described is in the form of tablets, placed vertically in a pipe in which the water to be treated circulates. Although it involves a single pass system and the surface of the tablets of compound exposed to the water is a function of the flow rate of the water in the pipe, it does not comprise any means for intervening constantly and in real time on the amount of active chlorine released by the tablets. Actually, the relation which exists between the tablets and the water flow rate comes from the sole surface of the tablets in contact with the water. The higher the layer of water is in the pipe, the more immersed are the tablets.